1. Field of the Invention
The present invention relates to methods for transferring data between a source and a number of receiving data processing devices. More precisely, the present invention relates to the methods used in data transfer in a local area network (LAN) between a number of data processing devices, where the devices use a wireless link media (such as radio signals or infrared signals) for delivering data.
2. Background
CSMA (carrier sense multiple access) is a powerful class of access schemes in wireless local area networks (LANs). The advantage of CSMA is its high throughput, simplicity of operation, and relatively ease of hardware implementation. When the number of stations is not too large and there is no obstacle on the path of (wireless) signals, all stations can hear each other. In this case the performance of the system can reach the theoretical limit which is approximately 98% [TOBAJI80]. Unfortunately, in practice that, is not always the case and stations most often cannot hear each other directly, which results in a mode of operation commonly referred to as the hidden terminal case. In such a case, there is a higher chance of collision between stations. The throughput is degraded substantially. In the limit, when all terminals become hidden from each other (and hence have the potential of colliding with others) the operation become similar to the pure ALOHA [ABBRAMSON70] and the throughput limit becomes only 18%.
An environment in which there is a high chance of presence of hidden terminals is the case in which the communication media is diffused infrared (IR). IR signals do not penetrate through solid objects, and signals are reflected and bounced back on solid objects and that is in fact how they reach stations and fill an environment.
When there are hidden terminals, one can use number of solutions to overcome the degradation in throughput efficiency. One is the use of a busy tone signal. In this case, a separate band is allocated for the busy tone signal. When a station is transmitting data, it sends the busy tone on the dedicated channel. Each terminal which hears a busy signal, sends a busy signal of its own. In this case the busy signal occupies the entire environment and potentially can be heard by all terminals. This technique, while effective, cannot be used in all environments. The busy signal channel must be carved out of the total bandwidth. In addition, terminals must be equipped with rather accurate filters to separate the busy signal channel from the data channel. The circuitry to do this is rather complex which is translated in a higher production cost.
Another approach is the use of a class of protocols, collectively knows as reservation protocols (or Collision Avoidance protocols, CA). In. this case, a station, before sending its data, reserves the channel so that no other terminal can use it. After transmitting its data, the station releases the channel for others to use. There is always a possibility that the reservation signals of different terminals collide. One solution is to broadcast the reservation signals using a CSMA like protocol. This class of protocols collectively is called CSMA/CA. The winner of the reservation becomes the sole user of the channel to transmit its data.
One such reservation protocol is the RTS/CTS protocol [Bharghavan94] (Request To Send/Clear To Send. The protocol operates as follows
When a station has data to send, it first senses the channel (as in the CSMA protocol). If it does not detect any activity and it is not aware of a current transmission (due to a previous RTS), it send an RTS to its destination. PA1 The receiver, when it detects an RTS destined to it, and if it can receive the data, broadcasts a CTS with the sending station address recorded in it. PA1 All other stations (which are passively listening on the channel), when they detect an ongoing RTS, keep quiet (and refrain from transmitting data) till they hear an end of transmission. PA1 The sending station, once it receives a CTS from its destination, sends its data. PA1 At the end of data transmission, there is a handshake between the sender and the receiver through which they broadcast the end of data transmission. PA1 All other stations having realized the channel is free, can now bid to get hold of the channel, as described before.
RTS/CTS, while a very effective protocol, suffers from low throughput efficiency. While its efficiency is substantially higher than pure ALOHA (18%), it is less than the efficiency of CSMA.
There is therefore a need for an adaptive access protocol.